Fuel injection strategies in opposed-piston engines with multiple fuel injectors

ABSTRACT

In an opposed-piston engine, two or more fuel injectors are mounted to a cylinder for direct side injection into the cylinder. The injectors are controlled so as to inject either a single fuel pulse or a plurality of fuel pulses per cycle of engine operation in order to initiate combustion during varying engine speeds and operating conditions.

PRIORITY

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/654,340, filed Oct. 17, 2012, for “Fuel Injection Strategiesin Opposed-Piston Engines with Multiple Fuel Injectors,” which claimsthe benefit of U.S. provisional application for patent No. 61/628,248,filed Oct. 27, 2011, for “Fuel Injection Strategies in Opposed-PistonEngines with Multiple Fuel Injectors”.

BACKGROUND

The present disclosure relates to fuel injection strategies foropposed-piston engines with direct side injection. In particular, thepresent disclosure concerns fuel injection in opposed-piston cylindersequipped with multiple injectors.

Efficient combustion is a fundamental challenge inherent in dieselengine operation. This challenge is typically addressed in conventionaldiesel engines by injecting fuel from a central location on the firedeck directly into an appropriately shaped combustion chamber definedbetween the end surface of the piston and the fire deck.

In opposed-piston architecture, two pistons are disposed crown to crownin a cylinder where they move in opposition between top dead center(TDC) and bottom dead center (BDC) positions. The combustion chamber isdefined in the cylinder, between the end surfaces of the pistons as theyapproach TDC. Consequently, it is not possible to mount a fuel injectorin a central position facing the end surface of a piston in anopposed-piston engine. Instead, a fuel injector mounting site istypically located on the sidewall of the cylinder, between the TDClocations of the piston end surfaces. This results in the direct sideinjection configuration that is characteristic of opposed-piston enginedesign. That is to say, fuel is injected directly into the combustionchamber, through the sidewall of the cylinder.

A problem with direct side injection is that the injected fuel travelsradially or tangentially into the cylinder, transversely to the axis ofcharge air swirl, which can inhibit air/fuel mixing and result inincomplete and/or uneven combustion. Accordingly, it is desirable toimprove the fuel injection capabilities of opposed-piston engines withdirect side injection.

SUMMARY

In one aspect, the present disclosure is directed to a fuel injectionmethod and system for improved direct side injection in anopposed-piston engine so as to enhance air/fuel mixing and improvecombustion.

In another aspect, the present disclosure is directed to anopposed-piston engine including at least one cylinder equipped with twoor more fuel injectors for direct side injection into the cylinder. Eachof the injectors is controlled so as to inject either a single fuelpulse or a plurality of fuel pulses per cycle of engine operation inresponse to varying engine speeds and operating conditions.

In a further aspect, the present disclosure relates to injecting fuelfrom first and second fuel injectors into a combustion chamber of anopposed-piston engine defined between the end surfaces of opposedpistons as the pistons approach top dead center locations, such thatfuel injected by the first fuel injector includes at least a pilotinjection followed by a main injection, and fuel injected by the secondfuel injector includes at least a pilot injection followed by a maininjection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an opposed-piston engine cylinderwith a single fuel injector mounted for direct side injection intocylinder bore space between the end surfaces of a pair of opposedpistons.

FIG. 2 is a cross sectional view of an opposed-piston engine cylinderwith multiple fuel injectors mounted for direct side injection intocylinder bore space between the end surfaces of a pair of opposedpistons.

FIG. 3 is a schematic diagram of a preferred fuel injection system foran opposed-piston engine.

FIG. 4 is a graph showing injector actuation and injection rates for thesingle fuel injector construction of FIG. 1.

FIG. 5 is a graph showing injector actuations and injection ratesembodying a first control strategy for the multiple fuel injectorconstruction of FIG. 2.

FIG. 6 is a graph showing injector actuations and injection ratesembodying a second control strategy for the multiple fuel injectorconstruction of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates a cylinder for an opposed-piston engine, with thecylinder shown in cross section. The cylinder 10 has exhaust and intakeports 12 and 14 located in the vicinity of respective ends of thecylinder. An electrically-actuated fuel injector 20 has a nozzle 22seated in a hole 24 drilled through the sidewall of the cylinder 10. Thehole 24 is located between bore locations 26 and 27 corresponding torespective TDC positions of the end surfaces of a pair of opposedpistons (not shown) disposed in the cylinder bore. When the pistons areat or near their TDC positions, the fuel injector 20 is operated toinject a spray of fuel 30 into the cylinder bore space between thepiston end surfaces.

FIG. 2 is a cross sectional view of an opposed-piston engine cylinder 40with multiple electrically-actuated fuel injectors 50 mounted for directside injection into cylinder bore space between the end surfaces of apair of opposed pistons. Each fuel injector 50 has a nozzle 52 seated ina hole 54 drilled through the sidewall of the cylinder 50. The holes 54are located between bore locations corresponding to respective TDCpositions of the end surfaces of the pair of opposed pistons 58 and 59disposed in the cylinder bore. As the pistons 58 and 59 move toward TDC,turbulently-moving charge air is compressed between their end surfaces.When the pistons 58 and 59 are at or near TDC, one or each of the fuelinjectors 50 is operated to inject a respective spray of fuel 60 intocompressed charge in the cylinder bore space between the piston endsurfaces. The fuel is mixed into the compressed, turbulent charge airand the heat produced by compression causes the air/fuel mixture toignite.

A fuel injection system for an opposed-piston engine is shown inschematic diagram form in FIG. 3. The fuel injection system 100 includesan opposed-piston engine 103 having one or more cylinders. Each cylinderhas a construction corresponding to that of the cylinder 40 of FIG. 2;thus, each cylinder 40 has a pair of opposed pistons (not shown)disposed for opposed movement therein and multiple fuel injectors 50between the TDC locations of the pistons. For example, each cylinder hastwo fuel injectors 50. Preferably, fuel is fed to the fuel injectors 50from a fuel source 160 that includes a rail/accumulator mechanism towhich fuel is pumped from a reservoir. A fuel return manifold 161collects fuel from the fuel injectors 50 and the fuel source for returnto the reservoir.

An engine control unit (ECU) 162 receives data from a variety of sensors(not shown) that measure parameter values related to operatingconditions of the engine, the transmission, and so on. The ECU 162includes a fuel injection control mechanization that implements fuelinjection control procedures in response to measured parameter valuesobtained from the sensors. These control procedures cause the generationof output control signals that are coupled to an electronic multichannelinjector driver 165. In response to the control signals, the injectordriver 165 generates drive signals on separate dedicated channels tooperate the fuel injectors 50. In other words, the injector driver 165electronically enables the operation of each fuel injector 50independently of every other fuel injector 50.

Preferably, each fuel injector 50 includes or is associated with anelectrically-operated actuator (such as a solenoid) that operates theinjector. Preferably, the actuator is controlled by respective drivesignals produced by the injector driver 165. A representative drivesignal is illustrated by a pulse-width modulated (PWM) waveform 167, forexample. Each such waveform has a first edge 168 that sets an actuatorto a first state and a second edge 169 that resets the actuator from thefirst to a second state. Preferably, the first state opens the nozzle ofthe fuel injector 50, initiating emission of a pulse of fuel 60 (alsocalled “an injection”) into the cylinder 40; while the second statedoses the nozzle of the fuel injector 50, terminating the injection.Alternatively, the fuel injection system 100 may be equipped to respondto other types and/or shapes of drive signals.

Presuming that pulse-width modulated waveforms are used to operate thefuel injectors 50, the duration of any one injection emitted by a fuelinjector corresponds to the width of the drive pulse: a narrow pulseproduces an injection of short duration; a wide pulse produces aninjection of longer duration. The fuel injection system 100 is designedto operate each injector in a plurality of fuel injection modes thatinclude one or more injections. In a first mode of operation, a fuelinjector emits a single injection into a combustion chamber in order toinitiate combustion. In a second mode of operation, called a “splitinjection”, a fuel injector emits two or more injections into acombustion chamber to initiate combustion: at least one first,relatively narrow, injection (called a “pilot’ injection) is followed bya second, relatively longer, injection (called a “main injection”). Thepilot and main injections are separated by a time interval. In order toproduce split injections with well-defined shapes and precisely-timeddurations, the operations of the fuel injection system 100 are alsodesigned to observe a minimum time interval (called “an inter-pulse gap”or IPG) measured between successive edges of successive drive signalpulses. Failure to adhere to the inter-pulse gap can result inoverlapping injections or variability in the quantity of fuel injected.

The fuel injection system 100 is further designed to transition fromsingle to split injection, and from split to single injection, asrequired by engine operating conditions. Further, with multipleindependently-controlled fuel injectors for each cylinder, the design ofthe fuel injection system 100 enables transition from operating a singlefuel injector per cylinder to operating a plurality of fuel injectorsper cylinder, and from operating a plurality of fuel injectors percylinder to operating a single fuel injector per cylinder, as requiredby engine operating conditions.

Fuel Injection with a Single Injector Per Cylinder:

With reference to FIGS. 1 and 4, an opposed-piston engine in which eachcylinder has only a single fuel injector is constrained in that acertain orifice area is necessary in order to introduce a fuel charge ina given number of crank angle degrees at a particular engine speed. Thislimitation can be detrimental to spray quality and may adversely limitminimum pilot fuel available. Further, a single injector actuated by asolenoid has limitations in that an IPG must be observed in order toassure the previous injection has completed and residual magnetism inthe solenoid has decayed. A single injector actuation (drive) signalcurve 180 and fuel injection rate curve 182 are shown in FIG. 4. Thegraph shows a split injection including two pilot injections 182 p 1,182 p 2, and a main injection 182 m. A first actuation pulse 180 a 1starts at 1 millisecond (msec) and is followed by pilot injection 182 p1 approximately 0.5 msec later. Following the end of the first pilotinjection 182 p 1, a second actuation pulse 180 a 2 starts at 2 msec.The rising edge of the second actuation pulse 180 a 2 occurs about 0.75msec after the falling edge of the first actuation pulse 180 a 1.Presume that 0.75 msec is the IPG. If the IPG is disregarded and thesecond actuation pulse 180 a 2 starts prior to the end of the firstinjection 182 p 1, then the quantity of injected fuel and timing of thesecond pilot injection 182 p 2 can deviate from intended values. This isdue to reduced injection response time on the second pilot injection 182p 2 resulting in errors in both timing and quantity. Manifestly theminimum interval imposed by the IPG limits the number of injections thatcan be applied in a given crank angle by a single injector.

Fuel Injection with Multiple Injectors Per Cylinder:

With reference to FIGS. 2 and 5, an opposed-piston engine includes atleast one cylinder equipped with multiple injectors actuatedsequentially and/or simultaneously. For example, as per FIG. 5, two fuelinjectors 50 are operated according to the actuation and injectioncurves shown in FIG. 5. There are three pilot injections shown, two (192p 1 and 192 p 2) from a first of the fuel injectors 50, and one (193 p)from the second fuel injector. Presume that the two actuation pulses 190a 1 and 190 a 2 that cause the two pilot injections 192 p 1 and 192 p 2are subject to the same IPG as the fuel injector of FIG. 1 (0.75 msec).The actuation pulse 191 a causes the third pilot injection 193 p tooccur between the two pilot injections 190 a 1 and 190 a 2, thusproviding three tightly-grouped pilot injections occurring in the time asingle injector can provide a maximum of two pilot injections. Thisresults in a maximum number of injections, N_(max), in a given time. Forexample, using two injectors N_(max) is given by:N_(max)=N_(1 max)+(N_(1max)−1), where N_(1 max) is the maximum number ofinjections from one fuel injector, under the constraint of a minimuminterval. With additional injectors and overlapping injections providedby multiple fuel injectors, greater numbers of injections can beachieved in a given time than can be with a single fuel injector.Because the fuel injectors are disposed at different circumferentialpositions on the periphery of the cylinder, they inject into differentregions of the combustion chamber, and available air is not an issuewith this method. The main injections 192 m+193 m produce an aggregatemain injection as shown in FIG. 5 that results in substantiallysimultaneous actuation of both injectors 50, as indicated by thealignment of the two main actuation pulses 190 m and 191 m. This doublesthe rate of injection into the combustion chamber and cuts the durationof the aggregated main injection in half.

An example of using multiple injectors in order to rate-shape theinjected quantity of an aggregated main injection is shown in FIG. 6. Asin FIG. 5, injection includes staggered split injections from two fuelinjectors that results in three tightly grouped pilot injections. Thegroup of pilot injections is followed by a rate-shaped aggregate maininjection 200 m. As shown, the second fuel injector is first actuated tocreate the first portion 200 m 1 of the main injection. After anappropriate delay, the first fuel injector is also actuated so that bothfuel injectors introduce fuel simultaneously. As is evident in FIG. 6,fuel is injected at a first rate during the first portion 200 m 1, andthe rate of injection increases in the second portion 200 m 2. The rateof injection is thereby “shaped”. Relative shifts of the main actuationpulses and/or changes in their durations can produce different shapes inthe aggregate main injection. The ability to rate-shape reducespre-mixed spikes and allows for much smoother combustion and reducedemissions.

In FIG. 6, a post injection 210 is shown following the aggregate maininjection 200 m. The post injection 210 can be produced by actuating thefirst injector alone, thus providing a minimal fuel charge for the postinjection. If desired, multiple post injections can be applied bymultiple actuations of either injector or by sequential actuation ofboth injectors similar to that described for the grouped pilotinjections.

As should be evident, direct side injection in an opposed-piston enginehaving a plurality of electronically-controlled fuel injectors locatedin the same cylinder allows for multiple degrees of freedom notavailable with a single fuel injector. Injections can be simultaneous,sequential or staggered thus affording very closely spaced injectionsand rate shaping. These extra degrees of freedom offer enhancedperformance and emission possibilities. Using multiple fuel injectorsallows for the injector nozzle holes to be optimally sized with respectto spray characteristics while at the same time providing theflexibility of being able to increase the number of holes available toallow for rapid introduction of the fuel charge. Conversely,independently controlled fuel injectors allow for decreasing the numberof holes at low fuel demand in order to lengthen injection time and tosmooth combustion. Having the ability to actuate a single injector ormultiple injectors in a given cycle allows the use of higher railpressure at lower quantities, providing superior spray quality incomparison to that available from a single injector having high flowproperties. Furthermore, by actuating multiple injectors sequentially atvarious positions around the periphery it is possible to have manyinjections of varying or similar quantity introduced into the cylinderat various locations and with full authority regarding the timing ofsaid injections.

Although fuel injection strategies for opposed-piston engines havedescribed with reference to representative embodiments, it should beunderstood that various modifications can be made without departing fromthe underlying principles. Accordingly, the patent protection to beafforded these strategies is limited only by the following claims.

The invention claimed is:
 1. A method of operating an opposed-pistonengine with fuel injectors located for direct side injection of fuelinto a cylinder of the opposed-piston engine, in which a pair of pistonsis disposed crown to crown in the cylinder, the method comprising:causing the pair of opposed pistons to move in the cylinder; compressingcharge air between end surfaces of the pair of opposed pistons as theymove toward top dead center positions in the cylinder; injecting fuelfrom a first fuel injector and a second fuel injector in response tosuccessive drive signal pulses; the fuel being injected into acombustion chamber defined in the cylinder between the end surfaces ofthe pair of opposed pistons as the pair of opposed pistons approach topdead center locations, such that fuel injected by the first fuelinjector includes at least a pilot injection followed by a maininjection, and fuel injected by the second fuel injector includes atleast a pilot injection followed by a main injection; maintaining aninter-pulse gap between successive edges of the drive signal pulses thatcauses each pilot injection to be separated from its following maininjection by at least a minimum time interval; and, causing the fuel toignite in response to heat produced by compression of the charge air. 2.The method of claim 1, wherein the pilot injections injected by thefirst and second fuel injectors are offset by an amount of time lessthan the inter-pulse gap.
 3. The method of claim 2, wherein the maininjections injected by the first and second fuel injectors aresubstantially simultaneous.
 4. The method of claim 1, wherein the fuelinjected by the first fuel injector includes a second pilot injectiondelayed from the pilot injection by at least the inter-pulse gap, andthe main injection is delayed from the second pilot injection.
 5. Themethod of claim 4, wherein the pilot injection injected by the secondfuel injector occurs in time between the pilot injection and the secondpilot injection of the first fuel injector.
 6. The method of claim 5,wherein the main injections of the first and second fuel injectors occursubstantially simultaneously in time.
 7. The method of claim 5, whereinthe main injections of the first and second fuel injectors overlap intime.
 8. The method of claim 1, wherein the fuel injected by the firstfuel injector further includes at least one post injection following themain injection.
 9. The method of claim 1, wherein the fuel injected bythe second fuel injector further includes at least one post injectionfollowing the main injection.